This invention relates to an optical receiver and, in particular, relates to an optical receiver having a dispersion compensation function for compensating a waveform degraded by dispersion in an optical fiber.
Recent drastically increasing data traffic in networks represented by the Internet has imposed requirement for development of optical communication network that allows large-capacity communication. Such a large-capacity optical communication network is achieved by using wavelength division multiplexing (WDM) technology.
The WDM is a technique that multiplexes dozens of wavelengths to transmit an optical signal in a single optical fiber. Additionally, optical amplifiers or regenerators are used to achieve long distance transmission over several-hundred kilometers. Currently, wavelength division multiplexing systems for 10 Gbit/s per wavelength have come into practical use.
To design a wavelength division multiplexing system using the WDM, a dispersion compensator (DC) is necessary. The DC compensates for waveform degradation caused by chromatic dispersion in an optical fiber. For example, to compensate for chromatic dispersion in a single mode fiber (SMF) having a length of 80 km and chromatic dispersion of +20 ps/nm/km in a communication band of 1.55 μm, dispersion compensation in the amount of approximately −1600 ps/nm is required.
In specific designing a wavelength division multiplexing system, to correct an optical signal traveling through an optical fiber to have an optimum waveform, a value of dispersion compensation different from the foregoing amount of dispersion compensation may be selected considering chirping of the optical signal and nonlinear effect of the optical fiber. In an SMF, a zero dispersion wavelength, at which the wavelength dispersion becomes zero, is 1.3 micrometer (μm).
For optical fibers, there are some kinds of fibers such as dispersion-shifted fiber (DSF) in addition to the SMF. The dispersion-shifted fiber is an optical fiber in which the amount of dispersion in an optical signal wavelength is reduced by shifting the zero dispersion wavelength to 1.55 μm, which is the waveband of the optical signal.
To determine the amounts of dispersion compensation appropriate for these optical fibers, the dispersion value in an optical fiber channel to be applied is measured or estimated, and a DC having an appropriate dispersion compensation value is installed in an optical transfer apparatus. Currently, commonly-used DCs are fixed DCs such as dispersion compensating fibers (DCFs), in which the compensation value is fixed.
In using a fixed DC, however, a problem occurs that a large stock of various DCs is required depending on the guaranteed value.
Moreover, transceivers applicable to 40 Gbit/s or more have been studied recently; optical transmission at such a high transmission rate causes a spectrum width to be stretched. Consequently, there arise problems: for example, a difference (residual dispersion) between collective compensation of a WDM signal by a DCF and the optimum compensation value in each wavelength, a slight difference in compensated dispersion value such as seasonal variation in fiber dispersion characteristics caused by change in ambient temperature of the fiber, and significant effect to transmission characteristics by polarization mode dispersion (PMD).
To solve these problems, tunable dispersion compensators have been studied.
JP 2000-511655 A discloses tunable dispersion compensators that compensate for chromatic dispersion in the optical domain includes a tunable dispersion compensating apparatus using a virtual image phase array (VIPA). JP2004-191521A discloses tunable dispersion compensators that include a tunable dispersion compensator using fiber Bragg grating (FBG) or an etalon.
JP 2007-274022 A discloses a technique of electric dispersion compensator is also known as polarization mode dispersion compensator that converts an optical signal to an electric signal and corrects the waveform in the electric domain. It is known that such dispersion compensation in the electric domain is effective to chromatic dispersion.
The compensated dispersion value in the tunable dispersion compensator is changed by controlling physical mirror position or by controlling temperature of the device or adjusting parameters in the electric circuits. Such a tunable dispersion compensator typically requires time on the order of seconds to stabilize the compensated dispersion value.
The dispersion value to be compensated is different depending on the transmission path or the wavelength; accurate estimation before receipt of the signal is difficult. Hence, the compensated dispersion value should be determined while changing the dispersion value of the DC within a certain range and observing the main signal state at each dispersion value.
JP 2007-060583 A discloses a method that has been conventionally proposed is a method that controls a dispersion compensator based on the clock signal level on the receiving side to control a delayed interferometer and the DC concurrently.
As described in JP 2007-060583 A, to determine an appropriate compensated dispersion value, a method has been known that changes the dispersion value for the tunable DC within a certain range. In JP 2007-060583 A, a technique is disclosed that monitors clock signal intensity from the reproducing circuit disposed at a stage subsequent to the tunable dispersion compensator while changing the dispersion value and determines the compensated dispersion value at the center value between two dispersion values at which the clock signal intensity reaches the peak, achieving efficient setting of the dispersion value.